Light emitting element drive apparatus and portable apparatus using same

ABSTRACT

A light emitting element drive apparatus capable of outputting the lowest voltage satisfying drive conditions and having high light emitting efficiency and low power loss, and a portable apparatus using the same, comprising an LED drive apparatus to which LEDs of different drive voltages required for emitting light are connected in parallel and driving one or more LEDs, wherein the LED drive apparatus has drive circuits connected to the corresponding LEDs among a plurality of LEDs and driving the corresponding LEDs with luminances based on set values and power supply circuits for deciding a drive voltage value required for the highest light emission among one or more LEDs driven to emit light based on drive states of drive circuits (for example terminal voltages of the current source) and supplying a drive voltage having at least the decided value to LEDs in parallel.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation Application of the patent application Ser. No.12/076,996, filed Mar. 26, 2008, which is a Continuation application of10/512,982, filed Oct. 29, 2004, now U.S. Pat. No. 7,365,718, which isbased on a National Stage Application of PCT/JP03/06735, filed May 29,2003, which in turn claims priority from Japanese Application No.:2002-160536, filed on May 31, 2002, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a light emitting element driveapparatus for driving a plurality of light emitting elements ofdifferent drive voltages and a portable apparatus using the same.

BACKGROUND ART

A portable apparatus, such as a mobile phone, is provided with lightemitting diodes (hereinafter described as LEDs) as a plurality of lightemitting elements of different light emission colors for emitting lightas backlight of an image display comprised of, for example, a liquidcrystal device (LCD) or displaying an incoming call etc.

Current portable apparatuses are generally provided with red (R) LEDs,green (G) LEDs, blue (B) LEDs, and white LEDs.

These various types of LEDs have different forward voltages (Vf). Forexample, the forward voltage Vfr of the red LEDs is set at approximately2.0V, the forward voltages Vfg and Vfb of the green and blue LEDs areset at approximately 3V, and the forward voltage Vfw of the white LEDsis set at approximately 3.5V.

Portable apparatuses mounting various types of LEDs having differentforward voltages in this way have a LED drive apparatus for drivingthese LEDs.

The output voltage in this LED drive apparatus is set by selecting avalue satisfying the forward voltage of the maximum value in order tohandle the various types of LEDs having different forward voltages. Forexample, when red LEDs having the forward voltage Vfr of 2.0V and whiteLEDs having the forward voltage Vfw of 3.5V are driven by the same powersupply, the output voltage of the LED drive apparatus is generally fixedto 4.5V to 5.0V by considering the variation of the voltage required forthe constant current source and the forward voltage Vfw of the whiteLEDs.

When using a LED drive apparatus having the output voltage matched withthe LEDs having the highest forward voltage, however, for example, redLEDs having a low forward voltage will be driven by a voltage higherthan the required drive voltage by 2.0V. As a result, there is anaccompanying very large power loss.

Further, as explained above, the apparatus is designed by including anoperating margin considering the variation of the LEDs having the highforward voltage. This operating margin becomes one of the factors ofpower loss.

This problem of power loss lowers the light emitting efficiency of theLEDs remarkably. A portable apparatus is driven by batteries due to itsportability, so this power loss will shorten the actual usage time ofthe portable apparatus.

For this reason, in conventional LED drive circuits, studies are beingmade for the purpose of raising the efficiency of the charge pump orDC-DC converter serving as the power supply. However, the efficiency ofthese circuits has already exceeded 90%. Therefore it becomes difficultto extend the actual usage time even if the efficiency is raised morethan this.

On the other hand, there is a method of connecting a few LEDs in seriesand driving them by a boosted power supply as a means for solving theabove problem.

By using this method, the output of the LED drive circuit is controlledto the voltage of the required lowest limit, so a high efficiency (highlight emitting efficiency) can be expected.

However, this method involves the following problems.

First, since the output voltage becomes high, a high voltage resistanceprocess becomes necessary.

Second, for an output within the voltage resistance, driving three tofour LEDs is the limit.

Third, the LEDs are connected in series, so independent control of theLEDs is difficult.

The third problem is especially large. The function of “many LEDsemitting light in various ways” expected from portable apparatuses inrecent years is not satisfied.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a light emittingelement drive apparatus not requiring a high voltage resistance process,capable of increasing the light emitting elements that can be driven andcapable of independently controlling each of a plurality of lightemitting elements, capable of constantly outputting the lowest voltagesatisfying the drive conditions even if individually adjusting theluminances of the plurality of light emitting elements and even ifsimultaneously driving a plurality of light emitting elements ofdifferent drive voltages, and having a high light emitting efficiencyand low power loss and a portable apparatus using the same.

To attain the above object, a first aspect of the present invention is alight emitting element drive apparatus wherein a plurality of lightemitting elements of different drive voltages required for emittinglight are connected in parallel and one or more light emitting elementsamong the plurality of light emitting elements are driven, comprising aplurality of drive circuits connected to corresponding light emittingelements of the plurality of light emitting elements and driving thecorresponding light emitting elements with luminances based on setvalues, a decision circuit for deciding a drive voltage value requiredfor the highest light emission among one or more light emitting elementsdriven to emit light based on drive states of the plurality of drivecircuits, and a power supply circuit for supplying a drive voltage tothe plurality of light emitting elements in response to the decisionresult of the decision circuit.

A second aspect of the present invention is a portable apparatus havinga battery as a power supply voltage source, comprising a plurality oflight emitting elements of different drive voltages required for thelight emission, at least one illuminated portion illuminated by thelight emitting element, and a light emitting element drive apparatus towhich the plurality of light emitting elements is connected in paralleland driving one or more light emitting elements among the plurality oflight emitting elements, wherein the light emitting element driveapparatus includes a plurality of drive circuits connected tocorresponding light emitting elements of the plurality of light emittingelements and driving the corresponding light emitting elements withluminances based on set values, a decision circuit for deciding a drivevoltage value required for the highest light emission among one or morelight emitting elements driven to emit light based on drive states ofthe plurality of drive circuits, and a power supply circuit forsupplying a power supply voltage as the drive voltage to the pluralityof light emitting elements in response to the decision result of thedecision circuit.

In the present invention, the power supply circuit fixes the outputdrive voltage of the power supply circuit to a predetermined set voltageirrespective of the drive states of the light emitting elements whenreceiving a predetermined flash operation instruction command.

In the present invention, the power supply circuit supplies a suppliedpower supply voltage as the drive voltage to the plurality of lightemitting elements when the value of the power supply voltage is largerthan the voltage value decided by the decision circuit.

In the present invention, the power supply circuit down-converts asupplied power supply voltage to any value down to the decided voltagevalue when the value of the power supply voltage is larger than thevoltage value decided by the decision circuit and supplies thedown-converted power supply voltage as the drive voltage to theplurality of light emitting elements.

In the present invention, the power supply voltage boosts a suppliedpower supply voltage to at least the decided voltage value when thevalue of the power supply voltage is smaller than the voltage valuedecided by the decision circuit and supplies the boosted power supplyvoltage as the drive voltage to the plurality of light emittingelements.

In the present invention, the power supply circuit down-converts asupplied power supply voltage to any value down to the decided voltagevalue and supplies the down-converted power supply voltage as the drivevoltage to the plurality of light emitting elements when the value ofthe power supply voltage is larger than the voltage value decided by thedecision circuit, supplies the supplied power supply voltage as thedrive voltage to the plurality of light emitting elements when thedecided voltage value and the value of the power supply voltages areapproximately the same, and boosts the supplied power supply voltage toat least the decided voltage value and supplies the boosted power supplyvoltage as the drive voltage to the plurality of light emitting elementswhen the value of the power supply voltage is smaller than the voltagevalue decided by the decision circuit.

In the present invention, the power supply circuit includes adown-converted power supply for down-converting a supplied power supplyvoltage to any value down to the drive voltage value required for thelight emission of the light emitting elements and supplying thedown-converted power supply voltage as the drive voltage to the targetlight emitting elements for the light emitting elements having values ofdrive voltages required for emitting light smaller than the value of thepower supply voltage.

In the present invention, the power supply circuit includes a boostedpower supply for boosting a supplied power supply voltage to at leastthe drive voltage value required for the light emission of the lightemitting elements and supplying the boosted power supply voltage as thedrive voltage to the target light emitting elements for the lightemitting elements having the values of drive voltages required foremitting light larger than the value of the power supply voltage.

According to the present invention, a light emission luminance is givenas the set value to a desired drive circuit from, for example, a hostapparatus.

Due to this, a corresponding light emitting element is driven so as toemit light with a luminance based on the set value from the drivecircuit.

At this time, the decision circuit decides the drive voltage valuerequired for the highest light emission among one or more light emittingelements driven in the light emission based on the drive states of aplurality of drive circuits.

Then, the power supply circuit supplies a drive voltage having at leastthe decided value to a plurality of light emitting elements in responseto the decision result of the decision circuit.

As a result, even if individually adjusting luminances of a plurality oflight emitting elements or simultaneously driving a plurality of lightemitting elements, the lowest voltage for satisfying the driveconditions can always be output. Accordingly, an improvement of thelight emitting efficiency can be achieved and, in addition, a reductionof the power loss can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of the basic configuration of a first embodiment of aLED (light emitting element) drive circuit according to the presentinvention.

FIG. 2 is a circuit diagram of an example of the configuration of acurrent drive circuit according to the present embodiment.

FIG. 3 is an abbreviated circuit diagram of a concrete example of theconfiguration of a boosted power supply, an error amplifier, a detectionvoltage output portion of a current drive circuit, and a power supplyvoltage source according to the present embodiment.

FIG. 4 is a circuit diagram of the principal configuration of a secondembodiment of a LED (light emitting element) drive circuit according tothe present invention.

FIG. 5 is a view of the basic configuration of a third embodiment of aLED (light emitting element) drive circuit according to the presentinvention.

FIG. 6 is a view for explaining the configuration and functions of aboosted/down-converted power supply according to a second embodiment.

FIG. 7 is a circuit diagram of the principal configuration of a fourthembodiment of a LED (light emitting element) drive circuit according tothe present invention.

FIG. 8 is a block diagram of an example of the configuration of aportable apparatus (terminal) employing a LED (light emitting element)drive circuit according to the present invention.

BEST MODE FOR WORKING THE INVENTION

Below, an explanation will be given of embodiments of the presentinvention with reference to the attached drawings.

First Embodiment

FIG. 1 is a view of the basic configuration of a first embodiment of aLED (light emitting element) drive circuit according to the presentinvention.

The present LED drive apparatus 10, as shown in FIG. 1, has a pluralityn (n is a positive integer) of LEDs 20-1 to 20-n having different drivevoltages required for emitting light, that is, forward voltages Vf,connected to it in parallel. These LEDs 20-1 to 20-n are driven with anyluminances (drive currents).

At this time, the LED drive apparatus 10 outputs the optimum voltage(for example, the lowest voltage) enabling driving at the set current ofthe LEDs having the maximum forward voltage Vf among a plurality of LEDsconnected in parallel from a terminal TVO to anodes of the LEDs 20-1 to20-n.

Note that the present LED drive apparatus 10 is supplied with a powersupply voltage Vcc by a power supply voltage source (PVS) 30, forexample, a battery, via a terminal TVI.

Below, the concrete configuration and functions of the LED driveapparatus 10 will be explained sequentially with reference to thedrawings.

The LED drive apparatus 10, as shown in FIG. 1, has a serial/parallelconversion circuit (S/P) 11, luminance (current) setting circuits (CSC)12-1 to 12-n, current drive circuits (CDRV) 13-1 to 13-n, an erroramplifier (EAMP) 14, and a boosted power supply (BST) 15.

Note that the error amplifier 14 and the boosted power supply 15 formthe decision circuit and the power supply circuit according to thepresent invention.

The serial/parallel conversion circuit 11 converts digital serial dataconcerning the current (luminance) value to drive the LEDs 20-1 to 20-nsupplied by the host apparatus, such as a not illustrated CPU, input viathe terminal TDI to parallel data and supplies digital data ID1 to IDnconcerning the current (luminance) value after the conversion to thecorresponding current setting circuits 12-1 to 12-n.

The current setting circuit 12-1 is configured by, for example, adigital/analog conversion circuit (DAC), converts the digital data ID1concerning the drive current (luminance) value supplied by theserial/parallel conversion circuit 11 to a current setting signal IA1 asthe analog signal, and supplies the same to the current drive circuit13-1.

The current setting circuit 12-2 is configured by, for example, adigital/analog conversion circuit (DAC), converts the digital data ID2concerning the drive current (luminance) value supplied by theserial/parallel conversion circuit 11 to a current setting signal IA2 asthe analog signal, and supplies the same to the current drive circuit13-2.

In the same way, the current setting circuit 12-n is configured by, forexample, a digital/analog conversion circuit (DAC), converts the digitaldata IDn concerning the drive current (luminance) value supplied by theserial/parallel conversion circuit 11 to a current setting signal IAn asthe analog signal, and supplies the same to the current drive circuit13-n.

The current drive circuit 13-1 has a current source connected to acathode of the LED 20-1 to be driven via the terminal TL1 and drives theLED 20-1 to emit light with a drive current in accordance with the setvalue of the current setting signal IA1 as the analog signal supplied bythe current setting circuit 12-1.

Further, the current drive circuit 13-1 outputs the voltage of theconnecting point of, for example, the terminal TL1 and the currentsource, that is, a voltage (VDRV-Vf1) obtained by subtracting theforward voltage Vf1 of the LED from the output drive voltage VDRV of theboosted power supply 15, as a detection voltage DV1 to the erroramplifier 14.

Note that this detection voltage DV1 becomes a signal indicating thedrive state in the current drive circuit 13-1, but the signal indicatingthe drive state is not limited to this voltage and may be aninter-terminal voltage, etc. of a resistor element 132 (FIG. 2) as well.

The current drive circuit 13-2 has a current source connected to thecathode of the LED 20-2 to be driven via the terminal TL2 and drives theLED 20-2 to emit light with the drive current in accordance with the setvalue of the current setting signal IA2 as the analog signal supplied bythe current setting circuit 12-2.

Further, the current drive circuit 13-2 outputs the voltage of theconnecting point of, for example, the terminal TL2 and the currentsource, that is, a voltage (VDRV-Vf2) obtained by subtracting theforward voltage Vf2 of the LED from the output drive voltage VDRV of theboosted power supply 15, as a detection voltage DV2 to the erroramplifier 14.

Note that this detection voltage DV2 becomes a signal indicating thedrive state in the current drive circuit 13-2, but the signal indicatingthe drive state is not limited to this voltage and may be aninter-terminal voltage, etc. of the resistor element 132 (FIG. 2) aswell.

In the same way, the current drive circuit 13-n has a current sourceconnected to the cathode of the LED 20-n to be driven via the terminalTLn and drives the LED 20-n to emit light with the drive current inaccordance with the set value of the current setting signal IAn as theanalog signal supplied by the current setting circuit 12-n.

Further, the current drive circuit 13-n outputs the voltage of theconnecting point of, for example, the terminal TLn and the currentsource, that is, a voltage (VDRV-Vfn) obtained by subtracting theforward voltage Vfn of the LED from the output drive voltage VDRV of theboosted power supply 15, as a detection voltage DVn to the erroramplifier 14.

Note that this detection voltage DVn becomes a signal indicating thedrive state in the current drive circuit 13-n, but the signal indicatingthe drive state is not limited to this voltage and may be aninter-terminal voltage etc. of the resistor element 132 (FIG. 2) aswell.

FIG. 2 is a circuit diagram of an example of the configuration of thecurrent drive circuit according to the present embodiment.

This current drive circuit 13 (−1 to −n), as shown in FIG. 2, has an-channel MOS (NMOS) transistor 131 as the current source, a senseresistor element 132, a current detection amplifier 133, and a currentcontrol amplifier 134.

A drain of the NMOS transistor 131 is connected to the cathode of thecorresponding LED 20 (−1 to −n) via the terminal TL (1 to n), the sourceis connected to one end of the resistor element 132 and a non-invertedinput (+) of the current detection amplifier 133, and a gate isconnected to the output of the current control amplifier 134.

The other end of the resistor element 132 is connected to a groundpotential GND and an inverted input (−) of the current detectionamplifier 133.

The inverted input (−) of the current control amplifier 134 is connectedto the output of the current detection amplifier 133, and thenon-inverted input (+) is connected to the supply line of the currentsetting signal IA (1 to n) as the analog signal by the current settingcircuit 12 (−1 to −n).

The current drive circuit 13(−1 to −n) of FIG. 2 drives the gate of theNMOS transistor 131 by the output of the current control amplifier 134,detects the current flowing through the NMOS transistor 131 by the senseresistor element 132, and amplifies the detection by the currentdetection amplifier 133.

Then, the gate voltage of the NMOS transistor 131 is controlled by thecurrent control amplifier 134 so that the current flowing through theNMOS transistor 131 becomes the set current value by the current settingcircuit 12(−1 to −n).

By this, the LED 20 (−1 to −n) to be driven emits light with theluminance in accordance with the set current.

Further, in the current drive circuit 13 (−1 to −n), the drain of theNMOS transistor 131 is connected to the corresponding input terminal ofthe error amplifier 14 so that the drain voltage of the NMOS transistor131 as the current source is supplied as the signal DV (1 to n) to theerror amplifier 14.

Further, in the current drive circuit 13 (−1 to −n), the lowestoperation voltage of the NMOS transistor 131 as the current source andthe sense resistor element 132 is set at, for example, about 0.5V to 1Vaccording to the transistor size or resistance value.

The error amplifier 14 compares the detection voltages (drain voltage ofthe NMOS transistor 131) DV1 to DVn output from n number of currentdrive circuits 13-1 to 13-n with a reference voltage Vref and outputs asignal S14 in accordance with the difference between the smallestdetection voltage and the reference voltage Vref to the boosted powersupply 15.

Note that the smallest detection voltage employed in the error amplifier14 corresponds to the highest forward voltage Vf in other words.

The boosted power supply 15 is configured by, for example, a DC-DCconverter, performs DC-DC conversion so that the power supply voltageVcc from the power supply voltage source 30 supplied via the terminalTVI becomes a value in accordance with the output signal S14 of theerror amplifier 14, and supplies a drive voltage VDRV in parallel to nnumber of LEDs 20-1 to 20-n from the terminal TVO.

Note that an externally attached capacitor C10 is connected between theterminal TVO and the ground potential GND.

The LED drive apparatus 10 according to the present first embodiment isconfigured so that, for example, if the reference voltage Vref of theerror amplifier 14 is 1V, feedback is applied to the boosted powersupply so that the terminal voltage to which the cathode of the LEDhaving the maximum forward voltage Vf is connected, in other words, thedrain voltage of the NMOS transistor 131 configuring the current sourceof the current drive circuit, becomes 1V.

FIG. 3 is an abbreviated circuit diagram of a concrete example of theconfiguration of the boosted power supply 15, the error amplifier 14,the detection voltage output portion of the current drive circuit, andthe power supply voltage source 30 according to the present embodiment.

FIG. 3 illustrates only the current drive circuit 13-2 including theconfiguration of FIG. 2 for simplifying the drawing and shows only theNMOS transistor 131 as the current source and the sense resistor element132 for the other current drive circuits.

The boosted power supply 15, as shown in FIG. 3, has a comparator 151,an oscillator 152, a pre-driver 153, a p-channel MOS (PMOS) transistor154, and a NMOS transistor 155.

The comparator 151 has the inverted input (−) connected to the output ofthe error amplifier 14, the non-inverted input (+) connected to theoutput of the oscillator 152, and the output connected to the input ofthe pre-driver 153.

Further, the PMS transistor 154 has a drain connected to a terminal TVIsupplied with the power supply voltage Vcc by the power supply voltagesource 30, a source connected to an output terminal TVO of the drivevoltage, and a gate connected to the first drive terminal of thepre-driver 153.

The NMOS transistor 155 has a source connected to the ground potentialGND, a drain connected to the connecting point of the source of the PMOStransistor 154 and the terminal TVI, and a gate connected to the seconddrive terminal of the pre-driver 153.

The comparator 151 performs a comparison based on so-called PWM (pulsewidth modulation), concretely, a comparison between the output signalS14 of the error amplifier 14 and the oscillation signal of theoscillator 152, and outputs a signal 5151 in accordance with thecomparison result to the pre-driver 153.

The pre-driver 153 outputs drive signals SD1 and SD2 to the first driveterminal and/or from the second drive terminal to the gate of the PMOStransistor 154 and the gate of the NMOS transistor 155 in accordancewith the output signal 5151 of the comparator 151 and supplies the valueof the power supply voltage Vcc supplied from the terminal TVI as it is(through) or after adjustment from the terminal TVO as the drive voltageVDRV to the anodes of the LEDs 20-1 to 20-n in parallel.

Namely, as explained above, the boosted power supply 15 adjusts thevalue of the power supply voltage Vcc so that the terminal voltage(drain voltage of the NMOS transistor 131 configuring the current sourceof the current drive circuit) to which the cathode of the LED having themaximum forward voltage Vf is connected becomes the reference voltageVref set at the error amplifier 14 and outputs the same as the drivevoltage VDRV.

Further, the power supply voltage source 30, as shown in FIG. 3, has,for example, a lithium ion battery 301 and an inductor 302 connectedbetween a positive pole of the battery 301 and the terminal TVI of theLED drive device 10.

Next, the operation by the above configuration will be explained.

For example, digital serial data concerning the current (luminance)value for the LEDs 20-1 to 20-n to be driven in accordance with theoperation mode are input from the host apparatus to the serial/parallelconversion circuit 11 via the terminal TDI.

The serial/parallel conversion circuit 11 converts the supplied digitalserial data concerning the current (luminance) values to drive the LEDs20-1 to 20-n to parallel data. Then, the digital data ID1 to IDnconcerning the current (luminance) values after conversion are suppliedto the current setting circuits 12-1 to 12-n.

Note that the digital data ID1 to IDn also include information for notdriving the corresponding LEDs.

The current setting circuits 12-1 to 12-n convert the digital data ID1concerning the drive current (luminance) values supplied by theserial/parallel conversion circuit 11 to current setting signals IA1 toIan as the analog signals and supply them to corresponding current drivecircuits 13-1 to 13-n.

The current drive circuits 13-1 to 13-n drive the LEDs 20-1 to 20-n withthe drive currents in accordance with the set values of the currentsetting signals IA1 to IAn as the analog signals supplied by the currentsetting circuits 12-1 to 12-n. By this, the LEDs 20-1 to 20-n emit lightwith luminances in accordance with the set current values or are held inan off state.

Further, the current drive circuits 13-1 to 13-n output voltages of theconnecting points of the terminals TL1 to TLn and the current source,that is, voltages (VDRV-Vf1) to (VDRV-Vfn) obtained by subtracting theforward voltages Vf1 to Vfn of the LEDs from the output drive voltageVDRV of the boosted power supply 15, as the detection voltages DV1 toDVn to the error amplifier 14.

The error amplifier 14 compares the detection voltages (drain voltagesof the NMOS transistor 13) DV1 to DVn output from n number of currentdrive circuits 13-1 to 13-n with the reference voltage Vref. As theresult of the comparison, a signal S14 in accordance with the differencebetween the smallest detection voltage and the reference voltage Vref isoutput to the boosted power supply 15.

The boosted power supply 15 performs the DC-DC conversion so that thepower supply voltage Vcc from the power supply voltage source 30supplied via the terminal TVI becomes a value in accordance with theoutput signal S14 of the error amplifier 14. Then, the drive voltagesVDRV are supplied in parallel to n number of LEDs 20-1 to 20-n from theterminal TVO.

Concretely, the comparator 151 of the boosted power supply 15 comparesthe output signal S14 of the error amplifier 14 and the oscillationsignal of the oscillator 152 and outputs a signal 5151 in accordancewith the comparison result to the pre-driver 153.

The pre-driver 153 outputs the drive signals SD1 and SD2 to the firstdrive terminal and/or from the second drive terminal to the gate of thePMOS transistor 154 and the gate of the NMOS transistor 155 inaccordance with the output signal 5151 of the comparator 151. Due tothis, the values of the power supply voltage Vcc supplied from theterminal TVI are supplied in parallel to the anodes of the LEDs 20-1 to20-n from the terminal TVO as they are (through) or after adjustment(boosted).

Namely, the boosted power supply 15 adjusts the value of the powersupply voltage Vcc so that the voltage of the terminal to which thecathode of the LED having the maximum forward voltage Vf is connected(drain voltage of the NMOS transistor 131 configuring the current sourceof the current drive circuit) becomes the reference voltage Vref set inthe error amplifier 14 and outputs the same as the drive voltage VDRV.

The concrete operation of the boosted power supply 15 to the lightemission (turning on) of the LEDs of each color becomes as follows.

Note that, here, the LEDs 20-1 to 20-n include one or more red (R) LEDs,green (G) LEDs, blue (B) LEDs, and white LEDs.

The forward voltages of the colors of the LEDs are as follows.

The forward voltage Vfr of the red LEDs is set at 1.9V, the forwardvoltages Vfg and Vfb of the green and blue LEDs are set at approximately3.1V, and the forward voltage Vfw of the white LEDs is set at 3.5V.

Further, assuming that the power supply voltage source 30 is a lithiumion battery, the power supply voltage Vcc is used within a range of from3.2V to 4.2V.

Further, assume that the lowest operation voltage a required for thecurrent drive circuits 13-1 to 13-n of the LEDs is made 0.5V.

Further, in the following explanation, the term “through” indicates thatthe PMOS transistor 154 configuring the output stage of the boostedpower supply 15 is made on, and the NMOS transistor is made off (theDC-DC converter operates with 100% duty).

In order to turn on the white LEDs having the forward voltage Vfw of3.5V, the voltage required as the drive voltage VDRV is 4V (=3.5V+0.5V).

The operation of the boosted power supply 15 in this case becomes“through” where the power supply voltage Vcc is within a range of4.0V<Vcc<4.2V.

On the other hand, when the power supply voltage Vcc is within a rangeof 3.2V<Vcc<4.0V, the operation is for “boosting” to, for example, 4V.

In order to turn on the green LEDs and the blue LEDs having the forwardvoltages Vfg and Vfb of 3.1V, the voltage required as the drive voltageVDRV is 3.6V (=3.1V+0.5V).

The operation of the boosted power supply 15 in this case becomes“through” where the power supply voltage Vcc is within a range of3.6V<Vcc<4.2V.

On the other hand, when the power supply voltage Vcc is within the rangeof 3.2V<Vcc<3.6V, the operation is for “boosting” to, for example, 3.6V.

In order to turn on the red LEDs having the forward voltage Vfr of 1.9V,the voltage required as the drive voltage VDRV is 2.4V (=1.9V+0.5V).

The operation of the boosted power supply 15 in this case becomes“through” within the whole range since it is assumed that the powersupply voltage Vcc is within the range of 3.2V<Vcc<4.2V. In the firstembodiment, the down-converted power supply is not included.

That is, the operation of the boosted power supply 15 becomes “through”where the output of the boosted power supply 15 expected by applyingfeedback via the error amplifier 14 is less than the power supplyvoltage (battery voltage) Vcc, so the LEDs will be directly driven bythe power supply voltage Vcc. Here, the loss of the power will beconsidered.

For example, when turning on only the blue LEDs having the forwardvoltage Vfb of 3.1V when the power supply voltage Vcc is 4.0V, therequired drive voltage VDRV is 3.6V (=3.1V+0.5V), so (4.0V−3.6V)×(drivecurrent) becomes the loss.

As explained above, according to the present first embodiment, provisionis made of the current drive circuits 13-1 to 13-n having the currentsource connected to the cathodes of the LEDs 20-1 to 20-n as the drivetargets via the terminals TL1 to TLn, driving the LEDs 20-1 to 20-n withthe drive currents in accordance with the set values of the currentsetting signals IA1 to IAn to emit light, and outputting the voltages ofthe connecting points between the terminals TL1 to TLn and the currentsource as the detection voltages DV1 to DVn, the error amplifier 14 forcomparing the detection voltages DV1 to DVn output from n number ofcurrent drive circuits 13-1 to 13-n with the reference voltage Vref andoutputting the signal S14 in accordance with the difference between thesmallest detection voltage and the reference voltage Vref, and theboosted power supply 15 for performing the DC-DC conversion so that thepower supply voltage Vcc from the power supply voltage source 30supplied via the terminal TVI becomes a value in accordance with theoutput signal S14 of the error amplifier 14 and supplying the drivevoltages VDRV in parallel from the terminal TVO to n number of LEDs 20-1to 20-n, and therefore the boosted power supply 15 adjusts the value ofthe power supply voltage Vcc so that the voltage of the terminalconnected with the cathode of the LED having the maximum forward voltageVf becomes the reference voltage Vref set at the error amplifier 14 andcan output the same as the drive voltage VDRV.

As a result, not only is a high voltage resistance process unnecessary,the light emitting elements that can be driven are increased, and eachof the plurality of light emitting elements can be independentlycontrolled, but also the lowest voltage satisfying the drive conditionsalways can be output even if the luminances of a plurality of LEDs areindividually adjusted and even if a plurality of LEDs having differentforward voltages are driven simultaneously.

Accordingly, there are the advantages that an improvement of the lightemitting efficiency can be achieved and a reduction of the power losscan be achieved.

In actual computation, a conventional device has a light emittingefficiency of about 50%, but in a device according to the presentembodiment, a light emitting efficiency of about 70% can be realized.

Second Embodiment

FIG. 4 is a circuit diagram of the principal configuration of a secondembodiment of a LED (light emitting element) drive circuit according tothe present invention.

In FIG. 4, the same components as those of FIG. 1 are represented by thesame notations.

Further, in FIG. 4, for simplification of the drawing, only the LED 20-1is illustrated and only the current setting circuit 12-1 and the currentdrive circuit 13-1A are illustrated corresponding to this, and the othercurrent setting circuits 12-1 to 12-n and the current drive circuits13-2A to 13-nA not illustrated in FIG. 4 also have the sameconfigurations.

The LED drive apparatus 10A according to the present second embodimenthas the configuration for making (turning on) the LEDs of the firstembodiment emit light explained above plus a configuration for makingthem flash.

The LED drive apparatus 10A is concretely provided with an inverter 16,a 2-input AND gate 17, a second error amplifier 18, a switch circuit 19,and output voltage division use resistor elements R11 and R12.

Further, each current drive circuit 13-1A (to 13-nA) is provided with aNMOS transistor 135 having a source connected to the ground potentialGND, a drain connected to the output of the current control amplifier134, and a gate connected to the output of the AND gate 17.

Further, the serial/parallel conversion circuit 11A is supplied withdigital data indicating the drive current (luminance) value plus acommand indicating whether to perform a normal turning on operation or aflashing operation at the terminal TDI.

The serial/parallel conversion circuit 11A outputs a low level signalS11 to one input of the AND gate 17 and the switch circuit 19 in thecase of a command for a normal operation, and outputs a high levelsignal S11 in the case of a command for a flashing operation.

The input of the inverter 16 is connected to an input terminal TSYC of apulse-like synchronization signal SYNC supplied by a not illustratedexternal synchronization signal supply circuit (for example sound sourceIC), while the output is connected to the other input of the AND gate17. The output of the AND gate 17 is connected to the gate of the NMOStransistor 135 provided in each current drive circuit 13-1A (to 13-nA).

The resistor elements R11 and R12 are connected in series between theconnecting point of the source of the PMOS transistor 154 of the boostedpower supply 15 and the terminal TVO and the ground potential GND, andthe connecting point of the resistor elements R11 and R12 is connectedto the inverted input (−) of the second error amplifier 18.

The non-inverted input (+) of the second error amplifier 18 is suppliedwith the reference voltage from the voltage source VSref.

The switch circuit 19 has a fixed output terminal a and switch inputterminals b and c; the fixed output terminal a is connected to theinverted input (−) of the comparator 151 of the boosted power supply 15;the switch input terminal b is connected to the output of the firsterror amplifier 14; and the switch input terminal c is connected to theoutput of the second error amplifier 18.

When the switch circuit 19 receives the signal S11 from theserial/parallel conversion circuit 11A at the low level (normal turningon indication), it connects the fixed output terminal a and the switchinput terminal b and inputs the output signal S14 of the first erroramplifier 14 to the inverted input (−) of the comparator 151. In thiscase, since the signal S11 is at the low level, the output of the ANDgate 17 is held at the low level. Accordingly, the NMOS transistor 135provided in each current drive circuit 13-1A (to 13-nA) is held in theOFF state.

Namely, at the time of the normal turning on, in terms of the circuit,it becomes equivalent to the circuit according to the first embodimentexplained above.

The operation at the time of turning on is carried out in the same wayas in the case of the first embodiment. Accordingly, a detailedexplanation is omitted here.

When the switch circuit 19 receives the signal S11 from theserial/parallel conversion circuit 11A at the high level (flashingoperation indication), it connects the fixed output terminal a and theswitch input terminal c and inputs the output signal S18 of the seconderror amplifier 18 to the inverted input (−) of the comparator 151.

In this case, since the signal S11 is at the high level, the output ofthe AND gate 17 is switched to the high level and the low level inaccordance with the inversion signal of the synchronization signal SYC.

Accordingly, the NMOS transistor 135 provided in each current drivecircuit 13-1A (to 13-nA) becomes the ON state when the output of the ANDgate 17 is at the high level. At this time, the output of the currentcontrol amplifier 134 is connected to the ground potential, so the NMOStransistor 131 serving as the current source is held in the OFF state,and the corresponding LEDs 20-1 (to 20-n) are held in the non-lightemission state.

On the other hand, the NMOS transistor 135 provided in each of thecurrent drive circuits 13-1A (to 13-nA) becomes OFF when the output ofthe AND gate 17 is at the low level. At this time, the NMOS transistor131 serving as the current source is driven by the output of the currentcontrol amplifier 134, and the corresponding LEDs 20-1 (to 20-n) areheld in the light emitting state.

Namely, the LEDs 20-1 (to 20-n) perform the flashing operation.

At the time of this flashing operation, as explained above, the circuitconfiguration for applying feedback of the output voltage to the boostedpower supply 15 via the second error amplifier 18 is exhibited asexplained above.

Due to this, the output drive voltage VDRV of the boosted power supply15 is fixed to the voltage set inside not according to the operationstate of the LED.

In the second embodiment, at the time of the flashing operation, thereason for making the circuit configuration one for applying feedback ofthe output voltage to the boosted power supply 15 via the second erroramplifier 18 for fixing the output drive voltage VDRV at the voltage setinside not according to the operation state of the LED is as follows.

For example, when assuming an operation where the red LEDs and the blueLEDs flash alternately flash if pursuing efficiency, the operation isdesirably carried out so that the output voltage of the boosted powersupply is down-converted at the time of light emission of the red LEDsand the output voltage rises at the time of the light emission of theblue LEDs, thereby raising the efficiency of the entire system.

In actuality, however, due to fluctuation of the output of the boostedpower supply 15 in synchronization with the synchronization signal,generation of noise is a concern.

Therefore, at the time of a flashing operation, by applying feedback ofthe output voltage of the boosted power supply 15 via the second erroramplifier 18 and fixing the output drive voltage VDRV at the voltage setinside not according to the operation state of the LEDs, fluctuation ofthe output of the boosted power supply 15 at the time of the flashing ofthe LEDs is suppressed.

According to the second embodiment, at the time of the normal turning onoperation, in addition to an effect the same as that obtained by thefirst embodiment explained above, at the time of the flashing operation,there are the advantages that the fluctuation of the output of theboosted power supply 15 is suppressed and a stable flashing operationcan be carried out without the influence of noise.

Note that the second embodiment was configured to apply feedback to theoutput voltage of the boosted power supply 15 via the second erroramplifier 18 and fix the output drive voltage VDRV to the voltage setinside not according to the operation state of the LED when receiving apredetermined flashing operation instruction command, but when, forexample, there is no influence of noise due to the flashing operationwith a low frequency, it also is possible to configure the device so asto, in the same way as the normal turning on operation, apply feedbackvia the first error amplifier 14 and, when there is an influence ofnoise by the predetermined flashing operation with a high frequency,apply feedback to the boosted power supply 15 via the second erroramplifier 18 and fix the output drive voltage VDRV to the voltage setinside not according to the operation state of the LED.

Third Embodiment

FIG. 5 is a view of the basic configuration of a third embodiment of aLED (light emitting element) drive circuit according to the presentinvention.

The difference of the third embodiment from the above first embodimentresides in that a boosted/down-converted power supply (BDPS) 101including a down-conversion function in addition to a boosting functionis provided in place of the boosted power supply as the power supplycircuit for outputting the drive voltage.

FIG. 6 is a view for explaining the configuration and function of theboosted/down-converted power supply 101 according to the thirdembodiment.

In the example of FIG. 6, nine (n=9) LED's, LED 20-1 to LED 20-9 areprovided in parallel.

Among the nine LEDs, two LEDs 20-1 and 20-4 are red LEDs, two LEDS 20-2and 20-5 are green LEDs, two LEDs 20-3 and 20-6 are blue LEDs, and threeLEDs 20-7 to 20-9 are white LEDs.

Further, corresponding to the LEDs 20-1 to 20-9, the current settingcircuits 12-1 to 12-9 and the current drive circuits 13-1 to 13-9 areprovided; but in FIG. 6, for simplification of the drawing, only thecurrent setting circuit 12-1 and the current drive circuit 13-1 areillustrated.

The boosting/down-converting circuit 101, as shown in FIG. 6, has adown-converting power supply drive circuit 1011, a power supply throughcircuit drive circuit 1012, a boosted power supply drive circuit 1013, adown-converted power supply (DPS) 1014, a power supply through circuit(PTR) 1015, and a boosted power supply (BST) 1016.

The down-converted power supply drive circuit 1011 receives the outputsignal S14 of the error amplifier 14 and drives the down-converted powersupply 1014 when the value obtained by adding the lowest operationvoltage a (for example, 0.5V) required for the current drive circuits13-1 to 13-9 of the LEDs to the maximum forward voltage Vf of the drivenLEDs is smaller than the value of the power supply voltage Vcc from thepower supply voltage source 30.

The power supply through circuit drive circuit 1012 receives the outputsignal S14 of the error amplifier 14 and drives the power supply throughcircuit 1015 when the value obtained by adding the lowest operationvoltage a required for the current drive circuits 13-1 to 13-9 of theLEDs to the maximum forward voltage Vf of the driven LEDs is equal tothe value of the power supply voltage Vcc from the power supply voltagesource 30.

The boosted power supply drive circuit 1013 receives the output signalS14 of the error amplifier 14 and drives the boosted power supply 1016when the value obtained by adding the lowest operation voltage arequired for the current drive circuits 13-1 to 13-9 of the LEDs to themaximum forward voltage Vf of the driven LEDs is larger than the valueof the power supply voltage Vcc from the power supply voltage source 30.

The down-converted power supply 1014, when driven by the down-convertedpower supply drive circuit 1011, down-converts the power supply voltageVcc from the power supply voltage source 30 by exactly the predeterminedvoltage and outputs the down-converted voltage as the drive voltage VDRVfrom the TVO.

The power supply through circuit 1015, when driven by the power supplythrough circuit drive circuit 1012, passes the power supply voltage Vccfrom the power supply voltage source 30 as it is and outputs the same asthe drive voltage VDRV from the terminal TVO.

The boosted power supply 1016, when driven by the boosted power supplydrive circuit 1013, boosts the power supply voltage Vcc from the powersupply voltage source 30 by exactly the predetermined voltage andoutputs the boosted voltage as the drive voltage VDRV from the terminalTVO.

The concrete operation of the boosted/down-converted power supply 101with respect to light emission (turning on) of the LEDs of each color isas follows.

Note that the forward voltage of the LEDs of each color is as follows inthe same way as in the case of the first embodiment.

The forward voltage Vf of the red LEDs is set at 1.9V, the forwardvoltages Vfg and Vfb of the green and blue LEDs are set at approximately3.1V, and the forward voltage Vfw of the white LEDs is set at 3.5V.

Further, assume that the power supply voltage source 30 is a lithium ionbattery and the power supply voltage Vcc is used within the range offrom 3.2V to 4.2V.

Further, assume the lowest operation voltage a required for the currentdrive circuits 13-1 to 13-n of the LEDs is 0.5V.

In order to turn on the white LEDs having the forward voltage Vfw of3.5V, the voltage required as the drive voltage VDRV is 4V (=3.5V+0.5V).

The operation of the boosted/down-converted power supply 101 in thiscase becomes the “down-conversion” operation when the power supplyvoltage Vcc is within the range of 4.0V<Vcc<4.2V. Concretely, thedown-converted power supply 1014 is driven by the down-converted powersupply drive circuit 1011. Due to this, the power supply voltage Vcc isdown-converted to 4V or any value down to 4V and output as the drivevoltage DVRV from the terminal TVO.

When the power supply voltage Vcc is equal to the voltage required forthe drive, the operation becomes “through”. Concretely, the power supplythrough circuit drive circuit 102 drives the power supply throughcircuit 1015. Due to this, the power supply voltage Vcc of 4V is passedand output as the drive voltage VDRV from the terminal TVO.

On the other hand, when the power supply voltage Vcc is within the rangeof 3.2V<Vcc<4.0V, the operation becomes a “boosting” operation forraising the voltage to 4V. Concretely, the boosted power supply drivecircuit 1013 drives the boosted power supply 1016. Due to this, thepower supply voltage Vcc is raised to 4V or a value more than this andoutput as the drive voltage DVRV from the terminal TVO.

In order to turn on the green LEDs and the blue LEDs having the forwardvoltages Vfg and Vfb of 3.1V, the voltage required as the drive voltageVDRV is 3.6V (=3.1V+0.5V).

The operation of the boosted/down-converted power supply 101 in thiscase becomes the “down-conversion” operation when the power supplyvoltage Vcc is within the range of 3.6V<Vcc<4.2V. Concretely, thedown-converted power supply drive circuit 1011 drives the down-convertedpower supply 1014. Due to this, the power supply voltage Vcc isdown-converted to 3.6V or any value up to 3.6V and output as the drivevoltage DVRV from the terminal TVO.

When the power supply voltage Vcc is equal to the voltage required forthe drive, the operation becomes “through”. Concretely, the power supplythrough circuit drive circuit 102 drives the power supply throughcircuit 1015. Due to this, the power supply voltage Vcc of 3.6V ispassed and output as the drive voltage VDRV from the terminal TVO.

On the other hand, when the power supply voltage Vcc is within the rangeof 3.2V<Vcc<3.6V, the operation becomes the “boosting” operation forraising the voltage to 3.6V. Concretely, the boosted power supply drivecircuit 1013 drives the boosted power supply 1016. Due to this, thepower supply voltage Vcc is boosted to 3.6V or a value more than thatand output as the drive voltage DVRV from the terminal TVO.

In order to turn on the red LEDs having the forward voltage Vfr of 1.9V,the voltage required as the drive voltage VDRV is 2.4V (=1.9V+0.5V).

The operation of the boosted/down-converted power supply 101 in thiscase becomes the “down-conversion” operation in the entire range sinceit is assumed that the power supply voltage Vcc is within the range of3.2V<Vcc<4.2V.

Concretely, the down-converted power supply drive circuit 1011 drivesthe down-converted power supply 1014. Due to this, the power supplyvoltage Vcc is down-converted to 2.4V or any value down to 2.4V andoutput as the drive voltage DVRV from the terminal TVO.

That is, the operation of the boosted/down-converted power supply 101becomes the “down-conversion” when the output of the boosted powersupply 15 expected by applying feedback via the error amplifier 14 isless than the power supply voltage (battery voltage) Vcc and becomes“boosting” when the output of the boosted power supply 15 is the powersupply voltage Vcc or more.

According to the third embodiment, in comparison with the above firstembodiment, there are the advantages that a further improvement of thelight emitting efficiency can be achieved and, in addition, a reductionof the power loss can be achieved.

Note that, needless to say, it is also possible to apply a circuitdesigned for the flashing operation explained in the second embodimentand give noise measures to the LED drive device 10B according to thethird embodiment.

Fourth Embodiment

FIG. 7 is a circuit diagram of the principal configuration of a fourthembodiment of a LED (light emitting element) drive circuit according tothe present invention.

The difference of the fourth embodiment from the third embodimentresides in that in the case of the red LEDs 20-1 and 20-4, only adown-converted operation is possible, so an error amplifier 102 and adown-converted circuit 103 dedicated to the red LEDs 20-1 and 20-4 areprovided, and a drive voltage obtained by down-converting the powersupply voltage Vcc to 2.4V is separately supplied to the red LEDs 20-1and 20-4.

Accordingly, the error amplifier 14C is supplied with the detectionvoltage signals DV2, DV3, and DV5 to DV9 from the current drive circuits12-2, 12-3, and 12-5 to 12-9 corresponding to the green LEDs, the blueLEDs, and the white LEDs.

The rest of the configuration is the same as the configuration of FIG.6.

According to the fourth embodiment, in comparison with the thirdembodiment, the overall light emitting efficiency can be raised.

Note that, needless to say, it is also possible to apply a circuitdesigned for the flashing operation explained in the second embodimentand give noise measures to the LED drive device 10C according to thefourth embodiment.

Fifth Embodiment

FIG. 8 is a view for explaining a fifth embodiment of the presentinvention and a block diagram of an example of the configuration of aportable apparatus (terminal) to which the LED drive apparatusesaccording to the first to fourth embodiments explained above can beapplied.

The portable apparatus 40 is configured by, for example. a mobile phoneset and, as shown in FIG. 8, has a CPU 41, a first image display (DSP1)42, a second image display (DSP2) 43, an input device (INPT) 44, anincoming call display unit (DSP3) 45, a synchronization signal supplycircuit (SYNCSPL) 46, and a LED drive apparatus (LEDDRV) 47 having anyof the configurations of the first to fourth embodiments explainedabove.

Then, the first image display 42, the second image display 43, the inputdevice 44, and the incoming call display unit 45 form an illuminatedportion illuminated by the LEDs.

The CPU 41 controls the operation of the device based on the input datafrom the input device 44, controls the display of the first imagedisplay 42 and the second image display 43 when the power supply is ON,controls the drive of the synchronization signal supply circuit 46, andcontrols the supply of the current (luminance) set data and the flashingoperation command data, etc. in accordance with the operation mode tothe LED drive apparatus 47.

The first image display 42 functions as the main display unit of theportable apparatus 40 and is configured by a liquid crystal display ableto perform a color display.

The first image display 42 is provided near it, as the illumination usebacklight, with three white LEDs (LEDs 20-7 to 20-9 in the examples ofFIG. 6 and FIG. 7) connected in parallel with respect to the LED driveapparatus 47.

The first image display 42 displays, under the control of the CPU 41, aradio reception state, an icon menu, various types of images, andincoming other party telephone numbers and messages, etc. input by theinput device 44.

The second image display 43 functions as a sub-display unit of theportable apparatus 40 and is configured by a liquid crystal display.

The second image display 43 is provided near it, for illumination, withLEDs of the three colors of red, green, and blue (LEDs 20-1 to 20-3 or20-4 to 20-6 in the examples of FIG. 6 and FIG. 7) connected in parallelwith respect to the LED drive apparatus 47.

The second image display 43 displays, under the control of the CPU 41,the time, date, etc. At the time of an incoming call or outgoing call,the LED drive apparatus 47 turns on or flashes one color, or any twocolors or all colors of the LEDs among the three colors of LEDS.

The input device 44 has a power supply switch, a tenkey, etc. and hasnear it, for illumination, LEDs of three colors of red, green, and blue(LEDs 20-1 to 20-3 or 20-4 to 20-6 in the examples of FIG. 6 and FIG. 7)connected in parallel to the LED drive apparatus 47.

The input device 44 is illuminated by one color, or any two colors orall colors of the three colors of LEDs from the LED drive apparatus 47when the power supply is on under the control of the CPU 41.

The incoming call display unit 45 is provided with LEDs of three colorsof red, green, and blue (LEDs 20-1 to 20-3 or 20-4 to 20-6 in theexamples of FIG. 6 and FIG. 7) connected in parallel with respect to theLED drive apparatus 47.

In the incoming call display unit 45, the LED drive apparatus 47 turnson or flashes one color, or any two colors or all colors of LEDs amongthe three colors of LEDs at the time of an incoming call.

The synchronization signal supply circuit 46 is configured by a MIDI oranother sound source IC and supplies a synchronization signal SYNC used,for example, for a flashing operation to the LED drive apparatus 47under the control of the CPU 41.

Note that, for the power supply of the portable apparatus 40, in thesame way as in the cases of the first to fourth embodiments explainedabove, use is made of a lithium ion battery.

The portable apparatus 40 having such a configuration is carried by theuser in a state with a first portion provided with the first imagedisplay 42, the second image display 43, and the incoming call display45 and a second portion provided with the input device 44 folded up, forexample, by a hinge mechanism.

Then, when the user opens the first portion and the second portion, forexample, when the power supply switch is turned on, in order toilluminate the first image display 42 by the backlight, current(luminance) setting data for driving the white LEDs are supplied to theLED drive apparatus 47 by the CPU 41.

Due to this, the LED drive apparatus 47 drives the white LEDs andilluminates the first image display 42 bright white.

Further, at this time, the current (luminance) setting data for drivingthe green LED are supplied to the LED drive apparatus 47 by the CPU 41so as to illuminate, for example, the input device 44 by the green LEDs.

Due to this, the LED drive apparatus 47 drives the green LEDs andilluminates the input device 44 lightly green.

Further, in the state where the first portion and the second portion arefolded up while, for example, the power supply is on, if there is anincoming call, in order to turn on or make the incoming call displayunit 45 and the second image display 43 flash by, for example, the redLEDs, the CPU 41 supplies current (luminance) setting data for drivingthe red LEDs to the LED drive apparatus 47. Further, when the mode forperforming the flashing operation is set, control is performed so thatthe CPU 41 outputs flashing operation instruction command data to theLED drive apparatus 47, and the synchronization signal supply circuit 46supplies a synchronization signal SYNC to the LED drive apparatus 47.

Due to this, the LED drive apparatus 47 drives the red LEDs to turn onor make the incoming call display 45 and the second image display 43flash red.

The operation of the LED drive apparatus 47 at the time of each aboveoperation is the same as explained in the first to the fourthembodiments. The value of the power supply voltage Vcc is adjusted sothat the voltage of the terminal to which the cathode of the LED havingthe maximum forward voltage Vf is connected becomes the referencevoltage Vref set at the error amplifier 14 and is output as the drivevoltage VDRV.

Due to this, even if the luminances of the plurality of LEDs areindividually adjusted and even if the plurality of LEDs having differentforward voltages are simultaneously driven, the lowest voltagesatisfying the drive conditions is always output.

Accordingly, the light emitting efficiency is high, and the power lossis suppressed low.

Here, a detailed operation of the LED drive apparatus 47 will beomitted.

According to the portable apparatus 40 according to the fifthembodiment, there are the advantages that the lowest voltage satisfyingthe drive conditions can be always output for the illumination use LEDs,an improvement of the light emitting efficiency can be achieved, areduction of the power loss can be achieved, and accordingly the servicelife of the battery can be prolonged.

INDUSTRIAL CAPABILITY

The light emitting element drive apparatus according to the presentinvention and a portable apparatus using the same can always output thelowest voltage satisfying the drive conditions, and an improvement ofthe light emitting efficiency and a reduction of the power loss can beachieved even if the luminances of a plurality of LEDs are individuallyadjusted and even if a plurality of LEDs having different forwardvoltages are simultaneously driven, so they can be applied to a mobilephone set driven by a battery, etc.

LIST OF REFERENCES

-   10, 10A TO 10C . . . LED (light emitting element) drive circuits-   11 . . . serial/parallel conversion circuit (S/P)-   12-1 to 12-n . . . current setting circuit (CSC)-   13-1 to 13-n . . . current drive circuit (CDRV)-   131 . . . NMOS transistor-   132 . . . sense resistor element-   133 . . . current detection amplifier-   134 . . . current control amplifier-   14 . . . error amplifier (EAMP)-   15 . . . boosted/down-converted power supply (BST)-   151 . . . comparator-   152 . . . oscillator-   153 . . . pre-driver-   154 . . . PMOS transistor-   155, 156 . . . NMOS transistor-   16 . . . inverter-   17 . . . 2-input AND gate-   18 . . . second error amplifier-   19 . . . switch circuit-   101 . . . boosted/down-converted power supply-   102 . . . error amplifier-   103 . . . down-converted power supply (DPS)-   1011 . . . down-converted power supply drive circuit-   1012 . . . power supply through circuit drive circuit-   1013 . . . boosted power supply drive circuit-   1014 . . . down-converted power supply (DPS)-   1015 . . . power supply through circuit (PTR)-   1016 . . . boosted power supply (BST)-   20-1 to 20-n . . . LEDs-   30 . . . power supply voltage (PSV)-   40 . . . portable apparatus-   41 . . . CPU-   42 . . . first image display (DSP1)-   43 . . . second image display (DSP2)-   44 . . . input device (INPT)-   45 . . . incoming call display (DSP3)-   46 . . . synchronization signal supply circuit (SYNCSPL)-   47 . . . LED drive apparatus (LEDDRV)

The invention claimed is:
 1. A light emitting element drive device,comprising: drive circuits configured to drive corresponding lightemitting elements with luminances based on a set value of a signal; adecision circuit configured to determine whether a lowest drive voltageis larger than a reference voltage, the lowest drive voltage being avoltage required among one or more of the light emitting elements toregulate a connecting point voltage of the drive circuits and the lightemitting elements; a power supply circuit configured to control at leastsaid one or more of the drive circuits to supply the lowest drivevoltage, the lowest drive voltage driving said one or more of the lightemitting elements to emit light based on drive states of the drivecircuits; and a current control amplifier and a current setting circuit,the current setting circuit being configured to convert a currentsetting signal to a current value to drive said one or more of the lightemitting elements.
 2. The light emitting element drive device of claim1, wherein said one or more of the light emitting elements is drivableto emit light with the luminance in accordance with a set current. 3.Alight emitting element drive device comprising: drive circuits, each ofthe drive circuits being configured to drive light emitting elementsbased on a set current value; a decision circuit configured to decidewhether drive voltages of the drive circuits are smaller than areference voltage; and a power supply control circuit configured toboost a supply voltage for one or more of the light emitting elementswhen the decision circuit decides that at least one of the drivevoltages of the drive circuits is smaller than the reference voltage. 4.The light emitting element drive device of claim 3, wherein the drivevoltages of the drive circuits correspond with the voltages of theconnecting points of the drive circuits and said one or more of thelight emitting elements.
 5. The light emitting element drive device ofclaim 3, wherein the power supply control circuit sends a signal to apower supply circuit in order to boost a supply voltage of the powersupply circuit.
 6. The light emitting element drive device of claim 5,wherein the power supply control circuit is controllable by pulse widthmodulation.
 7. The light emitting element drive device of claim 3,wherein the power supply control circuit is controllable by pulse widthmodulation.
 8. The light emitting element drive device of claim 7,wherein each of the drive circuits is a current driver.
 9. The lightemitting element drive device of claim 3, wherein each of the drivecircuits is a current driver.
 10. The light emitting element drivedevice of claim 9, further comprising: a serial parallel conversioncircuit configured to convert serial data into parallel data, theparallel data being a value to drive said one or more of the lightemitting elements.
 11. The light emitting element drive device of claim9, further comprising: a current setting circuit configured to a converta current setting signal to the set current value to drive said one ormore of the light emitting elements.
 12. The light emitting elementdrive device of claim 11, further comprising: a serial parallelconversion circuit configured to convert serial data into parallel data,the parallel data corresponding to the current setting signal.
 13. Thelight emitting element drive device of claim 12, wherein the powersupply control circuit is controllable by pulse width modulation.
 14. Alight emitting element drive device comprising: one or more drivecircuits configured to drive one or more light emitting elements basedon a set current value; one or more terminals configured to receive atleast one voltage from the one or more light emitting elementscorresponding to each of the one or more drive circuits; a decisioncircuit configured to decide an optimum drive voltage based on thevoltage from the one or more light emitting elements, the optimum drivevoltage being a voltage to apply currents driven by the one or moredrive circuits to the one or more light emitting elements; a powersupply control circuit configured to control voltage to supply theoptimum drive voltage to the light emitting elements.
 15. The lightemitting element drive device of claim 14, wherein the power supplycontrol circuit is controllable by pulse width modulation.
 16. The lightemitting element drive device of claim 15, wherein each of the one ormore drive circuits is a current driver.
 17. The light emitting elementdrive device of claim 14, wherein each of the one or more drive circuitsis a current driver.
 18. The light emitting element drive device ofclaim 17, further comprising: a current setting circuit configured toconvert a current setting signal to a current value to drive the one ormore light emitting elements.
 19. The light emitting element drivedevice of claim 18, further comprising: a serial parallel conversioncircuit configured to convert serial data into parallel data, theparallel data corresponding to a current setting signal.
 20. The lightemitting element drive device of claim 18, wherein the power supplycontrol circuit is controllable by pulse width modulation.
 21. The lightemitting element drive device of claim 14, further comprising: a serialparallel conversion circuit configured to convert serial data intoparallel data, the parallel data being a value to drive the one or morelight emitting elements.
 22. The light emitting element drive device ofclaim 14, wherein the power supply control circuit sends a signal to apower supply circuit in order to boost the supply voltage from the powersupply circuit.
 23. The light emitting element drive device of claim 22,wherein the decision circuit decides the optimum drive voltage bycomparing a received voltage with a reference voltage.
 24. The lightemitting element drive device of claim 14, wherein the power supplycontrol circuit is controllable by pulse width modulation.
 25. The lightemitting element drive device of claim 14, wherein the decision circuitdecides the optimum drive voltage by comparing a received voltage with areference voltage.
 26. The light emitting element drive device of claim14, wherein each of the one or more drive circuits is connected to acorresponding terminal.